As an electrostatic latent image development for electrophotography, an one-component magnetic jumping development, an one-component non-magnetic contacting development, and a two-component development, in which development is performed by frictionally electrifying a toner, transporting a developer, and contacting with an electrostatic latent image, by mixing an insulating non-magnetic toner and magnetic carrier particles, are known so far.
Particularly attention has been paid gain to application of the two-component development to the color printer as a promising method in near future.
A particulate carrier, which is used in such two-component development, is usually produced by coating a magnetic carrier core material with an appropriate material in order to prevent from a toner-filming onto the surface of the carrier, to form a carrier-uniform surface, to elongate the lifetime of a developer, to prevent damage of a sensitizer by a carrier, to control charge quantity, or for other purposes.
Conventional resin-coated carriers are not, however, satisfactory in durability because the coat is easily exfoliated by an impact such as stirring applied when used or for other reasons.
On the other hand, as methods to solve the above-mentioned problems, a method to improve charge quantity by containing nigrosin in a carrier-coated resin is disclosed in Japanese Patent Laid-open Pub. No. Sho 53-100242, a method to improve fluidity by adding a fluidity-improving agent is disclosed in Japanese Patent Laid-open Pub. No. Sho 61-9661, and a method to prevent making the charging property uniform and being spent by adding one selected from a group consisting of electroconductive fine particles, inorganic filler particles, and a charge-controlling agent, is disclosed in Japanese Patent Laid-open Pub. No. Hei. 2-210365.
These methods, however, could not satisfy providing the carrier having both the good controlling charging characteristics (controlling charge polarity, adjusting charge quantity, and adjusting resistance), and the good durability by preventing external additives of a toner from being spent.
In view of the above situation, the inventors of the present invention have proposed to bury convex polyhedron magnetic particles or carbon black in the outermost layer of an electrophotographic carrier which is coated with a high molecular weight polyethylene resin obtained by a direct polymerization process as disclosed in Japanese Patent Laid-open Pub. No. Hei 10-171168, thereby providing an electrophotographic carrier which is well-balanced between charging characteristics and durability.
However, when such an electrophotographic carrier is used in applications as color printers, there is the case where magnetic powders or carbon black which are imperfectly buried are fallen off. As a result, this opens up the problem that there is the case of forming a dull color image.
While, Japanese Patent Laid-open Pub. No. Hei 9-54461 discloses a process for producing an electrophotographic carrier, the process being characterized in that conductive fine particles having a whiteness of 0.2 or less and a volumetric specific resistance of 1.times.10.sup.2 to 1.times.10.sup.11 .OMEGA..multidot.cm is added to a coating resin of an electrophotographic carrier when a polyolefin type resin is polymerized.
However, when the electrophotographic carrier obtained in such a production process is used at high temperature and high humidity for a long period of time, such a phenomenon is found that the conductive fine particles absorb water to decrease charge quantity.
The present invention has been made in view of the above problems and has an object of providing an electrophotographic carrier having the following characteristics, making use of excellent qualities of a carrier provided with a polyolefin in type resin coating.
i) The control of its charge quantity and the adjustment of its resistance can be respectively made at will. PA1 ii) It prevents external additives for a toner from being spent and hence has an excellent durability. PA1 iii) It scarcely causes the resultant color image to be dull even if it is used for a color developer. PA1 iv) It has an excellent moisture resistance.
Another object of the present invention is to provide a process for producing the aforementioned electrophotographic carrier in an efficient manner.
A further object of the present invention is to provide a developing agent for electrophotography using the aforementioned electrophotographic carrier.